Hydrogels are polymers that swell in water. The term, “swelling” refers to the uptake of a liquid by a gel with an increase in volume. Only those liquids that solvate a gel can cause swelling. The swelling of ionic hydrogel gels is influenced by pH and the presence of electrolytes. Microgels have a large molecular weight that generally cannot be measured by conventional methods because they are too large, and are composed of a polymer backbone and crosslinks. The crosslinks can be used to extend the molecular weight of a polymer if the ratio of crosslinker to non-crosslinker is low, and polymerization is confined below the gel point. Nevertheless, if the ratio of crosslinking monomer to non-crosslinking monomer is high enough, a gel is formed that while still being able to swell in a solvent, does not truly dissolve.
The viscosity behavior of crosslinked polyelectrolyte microgels has been understood in terms of a model based on close packed spheres. At low concentrations, no yield stress (where yield stress is defined as the applied stress which must be exceeded to make a structural fluid flow) and little viscosity is observed because the swollen microgels are not tightly packed. Above some minimum packing concentration the particles are viewed as being closely packed deformable particles and the viscosity builds tremendously. Yield behavior and viscosity only begins when the concentration is such that the particles become closely packed.
The solution viscosity behavior of thermoplastic polyurethane is that of a linear polymer in a random configuration. The viscosity is dependent on both the molecular weight of the polymer and the solvency of the dissolution media. A linear randomly coiled polymer will have an intrinsic viscosity at low concentration that relates to the volume of the polymer and to the molecular weight. At higher concentrations, entanglement of the random coils will increase the apparent viscosity of the solution. Solutions of linear polymers in a random configuration usually do not show yield behavior and are not able to suspend particles.
In characterizing these materials used as thickeners, emulsifiers and suspending aids, the response of these materials to stress and simple flow fields may be used to determine their material functions, such as viscosity and response to stress.
Mathematical models have been developed to describe these properties. Rheological measurements on gels or thickened and suspended materials define the structure and properties of the material and can be used to identify changes and characteristics of an improved material over that being currently used. To those skilled in the art, interpretation of the response of a polymeric dispersion or gel to stress and strain is highly indicative of the material.
There is currently a need for polymeric compositions having high yield point, yield stress properties, as well as high response to shear as well as a need for improved methods of making these polymer compositions. There is also a need for improved polymeric compositions for use as thickeners, emulsifiers, suspending aids, and pharmaceutical controlled release excipients.